1. Field of the Invention
The present invention relates to an image pickup apparatus. More specifically, it relates to an image pickup apparatus for scanning an original by a reflective mirror to acquire an image of the original.
2. Description of the Prior Art
An image pickup apparatus employing an image scanner or a television camera is well known as means for acquiring images of sheet-type or book-type originals such as documents and drawings. Resolution of a television camera is about 512 pixels.times.512 lines, while a higher level of resolution, e.g., 2048 pixels.times.2048 lines may be required for acquiring the. In order to implement an image pickup apparatus of such high resolution, a linear image sensor has generally been combined with some vertical scanning mechanical system.
Well-known examples of such an image pickup apparatus are disclosed in "An Anthropomorphic Retina-like Structure for Scene" reported by Giulio Sandini in "Computer Graphics and Image Processing" No. 4, 1980, pp. 365-372, "Optical Scanning Digitizers" reported by George Nagy in "Computer" May 1983, pp. 13-24 and "The Wang Professional Image Computer: A New Dimension to Personal and Office Computing" reported by Fredrick A. Wang et al., in "Processing of the IEEE" Vol. 72, No. 3, March 1984, pp. 300-311.
Japanese Patent Laying-Open Gazette Nos. 69173/1983 and 34279/1981 also disclose well-known examples of the aforementioned image pickup apparatus.
FIGS. 1 and 2 are block diagrams schematically showing the image pickup apparatuses disclosed in the aforementioned Japanese Patent Laying-Open Gazette Nos. 69173/1983 and 34279/1981.
Referring to FIG. 1, a lens 1 is provided above an original 10, to form a two-dimensional image of the original 10 on its image forming plane. The lens 1 is provided on its image forming plane with a linear image sensor 4. The linear image sensor 4 is mounted to a linear sliding mechanism 30, which is slidable in the vertical scanning direction as shown by an arrow A. The linear image sensor 4 is moved in the vertical scanning direction to sense the two-dimensional image of the original 10. Picture signals obtained by the linear image sensor 4 are sequentially stored in a frame memory 5, so that the stored contents are partially displayed as an image on a display unit 6.
On the other hand, the image pickup apparatus as shown in FIG. 2 is provided with a polygonal reflective mirror 40 in place of the linear sliding mechanism 30 as shown in FIG. 1. The polygonal reflective mirror 40 is rotated at a constant speed, to sequentially form the image of linear parts of an original 10, which linear parts are in parallel with the axis of rotation on a linear image sensor 4. Picture signals are thus obtained by vertical scanning through rotation of the polygonal reflective mirror 40 and main scanning through image sensing by the linear image sensor 4 to be stored in the frame memory 5, whose storage contents are partially displayed on the display unit 6.
The aforementioned conventional image pickup apparatuses have been disadvantageous in scanning speed for image acquisition as well as immediateness and full observableness of image display. In the image pickup apparatus as shown in FIG. 1, for example, the linear sliding mechanism 30 can perform vertical scanning while maintaining sufficient mechanical accuracy at about the maximum speed of 10 mm/sec. On the other hand, a pixel-to-pixel interval of the linear image sensor 4 is 13 .mu.m. If the scanning line interval on the image forming plane in vertical scanning is equalized to the said interval, the maximum scanning speed is 769 line/sec., and hence at least 2.66 seconds are required for entirely scanning one frame (e.g., 2048 lines).
However, in case where an image is displayed on an ordinary display unit such as a CRT requiring refreshment, the entire frames must be repeated at about 30 frame/sec. to prevent trouble such as flicker, as well known in the art of, e.g., television receivers. Thus, the scanning speed of the aforementioned linear sliding mechanism 30 is absolutely insufficient for preventing the flicker, and hence the picture signals are temporarily stored in the frame memory 5 to be read at a high speed for display. Further, even if the frame memory 5 is of mass storage which can store 2048 pixels.times.2048 lines, an ordinary display unit cannot collectively display the whole storage contents as an image, and hence an extremely high-priced display unit of high resolution is required for collective display.
When the display unit 6 as employed is an ordinary one, therefore, the storage contents of the frame memory 5 must be divided into numbers of parts to be sequentially displayed. After all, the conventional image pickup apparatus as shown in FIG. 1 can only partially observe an image under sensing once per 2.66 seconds, and has been extremely inconvenient in operation such as setting of an image sensing range and focus control.
On the other hand, the image pickup apparatus as shown in FIG. 2 has no such problem of the scanning speed since the polygonal reflective mirror 40 can be rotated at a sufficiently high speed. However, the operation speed of the linear image sensor 4 has such an upper limit that the number of scanning lines for image sensing is decreased, and hence the image cannot be sensed in high resolution. The speed of rotation of the polygonal reflective mirror 40 must inevitably be reduced to sense the image in high resolution, whereas, also in this case, the scanning cycle is too long and the ordinary display unit 6 can only partially observe the image similarly to the image pickup apparatus as shown in FIG. 1.
When the aforementioned conventional image pickup apparatus is applied to office automation or factory automation, a region to be noted (hereinafter referred to as a noted region) in an original is preferably specified to be scanned and inputted in high resolution in addition to continuous input of the entire image of the original.
However, the image pickup apparatus as shown in FIG. 1 has a long scanning cycle and can merely partially display the image, and hence the same is inferior in operability for specifying the noted region. Further, an access time for moving a scanned range to the noted region upon specification is not ignorable.
On the other hand, the noted region can be easily specified in the image pickup apparatus as shown in FIG. 2 by rotating the polygonal reflective mirror 40, whereas the noted region cannot be read in high resolution. Further, the polygonal reflective mirror 40 is generally of great mass and hence it is difficult to control the same to abruptly reduce its rotation speed from a state of high speed rotation in order to scan the noted region in high resolution. Thus, this image pickup apparatus is unsuitable for scanning an original with variable scanned ranges and resolution.